1. Field of the Invention
The present invention relates to a thin film transistor (TFT), and more particularly to a TFT having an asymmetrical lightly doped drain(LDD) structure, which can reduce a kink phenomenon which is an unstable operation characteristic of the TFT.
2. Description of the Prior Art
FIG. 1A shows a construction of a conventional top-gate coplanar self-aligned TFT having a non-LDD structure. In the TFT of FIG. 1A, source and drain regions 2 are formed on a transparent insulating substrate 1 by doping high-density impurities (n.sup.+). An active layer 3 is formed between the source and drain regions 2. On the upper surface of the active layer 3, a gate-insulating layer 4 is formed, and then a gate electrode 5 is formed on the gate-insulating layer 4.
FIG. 1B shows a construction of a conventional TFT having an LDD structure. In the LDD type TFT of FIG. 1B, source and drain regions 2 are formed on the substrate 1 by doping high-density impurities(n.sup.+), and LDD regions 6 are also formed on the substrate 1 by doping low-density impurities (n.sup.-).
An active layer 3 is formed between the LDD regions 6. 0n the upper surface of the active layer 3 is formed a gate-insulating layer 4, and then a gate electrode 5 is formed on the gate-insulating layer 4. FIG. 2 shows an equivalent circuit of the TFTs as shown in FIGS. 1A and 1B.
Meanwhile, according to the TFTs having the non-LDD structure and the LDD structure of FIG. 1A and FIG. 1B, a process of injecting ions of n.sup.+ or P.sup.+ type is performed in order to form source and drain regions 2. By such ion injection, junctions are formed between the source and drain regions 2 and the active layer 3, respectively.
FIG. 3 is a schematic diagram of the TFT having the non-LDD structure of FIG. 1A, showing the state that the gate and drain voltages V.sub.G and V.sub.D are applied to the corresponding electrodes. Referring to FIG. 3, junctions are formed between the source and drain regions 2 and the active layer 3, and thereby, as shown in FIG. 4A, a step energy band is formed at the junctions due to the difference of charge density between the source and drain regions 2 and the active layer 3.
In this case, if a drain voltage V.sub.D larger than OV (i.e., V.sub.D &gt;0) is applied to the drain region, the step energy band is inclined along the traveling path of the carriers in the doped semiconductor, which may be electrons or holes, in accordance with the applied voltage as shown in FIG. 4B. The inclination of the energy band as described above corresponds to an electric field, and thus the carriers are transferred in their own directions according to the electric field, which forms the electric current of the TFT.
That is, if a voltage V.sub.D larger than OV (i.e., V.sub.D &gt;0) is applied to the drain region, holes are transferred from the drain region to the source region. In this case, the holes are accumulated at the junction in the source region until the drain voltage reaches a predetermined level, because the transfer of the holes are obstructed due to the energy level difference formed at the junction. The holes accumulated at the junction as described above have effect on the electric field at the junction. Thereafter, if the drain voltage V.sub.D goes on increasing to reach the predetermined level surpassing the energy difference at the junction, an abrupt flow of drain current, which is defined as a kink phenomenon, occurs as shown in FIG. 5.
The kink phenomenon as described above makes the operation characteristic of the TFT unstable. Though a method of making the thickness of the active layer thinner is adopted presently to prevent occurrence of the kink phenomenon, there are difficulties in the working process thereof such as etching.